Among the techniques of recognizing a lane mark (white line) as painted on a road surface, there is, for example, a technique disclosed in Patent Document 1. In this white line detection method, such pixels experiencing significant lightness changes are selected in a picture image of the road surface as captured by an image pickup unit. Out of these pixels, those that are within a preset zone proximate to the car and whose coordinates are located on a straight line are extracted and the straight line is concluded to be an edge of the white line. Using, as a start point, the last one of the pixels that make up the above straight line, the next pixel is searched within a preset range of the picture image in a direction away from the car. The pixels as searched in such range are decided to be the edge of the next white line.
Patent Document 1:
    JP Patent Kokai Publication No. JP-A-7-302346Non-Patent Document 1:    Greg Welch and Gary Bishop, An Introduction to the Kalman Filter, UNC-Chapel Hill, TR95-041, Jul. 24, 2006